Simplify the following expression: $p = \dfrac{3}{7y} - \dfrac{1}{3y}$
Explanation: In order to subtract expressions, they must have a common denominator. The smallest common denominator is the least common multiple of $7y$ and $3y$ $\lcm(7y, 3y) = 21y$ $ p = \dfrac{3}{3} \cdot \dfrac{3}{7y} - \dfrac{7}{7} \cdot \dfrac{1}{3y} $ $p = \dfrac{9}{21y} - \dfrac{7}{21y}$ $p = \dfrac{9 -7}{21y}$ $p = \dfrac{2}{21y}$